Magnetic resonance imaging (MRI) is rapidly becoming the most important imaging technique in medicine. Despite their considerable expense, hundreds of MRI systems are now in use in medical facilities throughout the world. MRI systems utilize the magnetic properties of the chemical constituents in body tissue in order to obtain an image. Detailed images of both hard and soft tissues can be obtained using MRI. Soft tissues may be imaged without using invasive and sometimes risky contrast agents as often required with X-ray imaging systems. In fact, MRI can obtain images which were heretofore unobtainable using X-ray or other imaging techniques. Moreover, the magnetic and radio frequency fields used in MRI techniques are believed to be considerably safer for the patient than the ionizing radiation required for X-ray techniques. Thus, MRI is an extremely important new imaging technology.
Magnetic resonance techniques may also be used to perform spectroscopy. Such techniques, which analyze tissues by detecting magnetic resonance properties of their constituent chemicals are known as magnetic resonance spectroscopy (MRS). MRS may be used to perform non-invasive in-vivo tissue analysis in place of or as a supplement to the conventional biopsy. MRS can also be used to obtain biochemical information non-invasively from tissues not accessible to biopsy procedures. Thus, MRS is capable of providing information on tissue make-up not obtainable through X-ray or any other imaging technology.
The basic operating theory behind MRI and MRS is set forth in simplified form as follows. First, a constant, uniform magnetic field (B.sub.0) must be generated throughout the body area or tissue to be imaged or analyzed. This B.sub.0 field aligns the various magnetic moments or "spins" in the body's matter. By applying a transient magnetic field (B.sub.1) oriented perpendicularly to the B.sub.0 field the spins can be temporarily tilted out of alignment with the B.sub.0 field. As they return to their aligned state each different chemical component emits a characteristic energy which is typically in the RF band. This emission can be detected to form the basis for an image or a spectroscopic analysis. Most imaging systems detect the presence of hydrogen due to its prevalence in the body. However, other elements and compounds may be used for the basis of imaging.
Most often permanent supercooled magnets are used to generate the required B.sub.0 field. The magnetic field is generated by one or more solenoids made of a superconductive alloy wire which is cooled with liquid helium to a superconducting state. The solenoids are axially aligned and the field is generated in a cylindrical volume of space coextending with the center areas of the solenoids. For in-vivo applications, this area must be large enough to accommodate the body portion to be imaged or analyzed. A typical coil used for in-vivo applications has an inside diameter in the range of one to three feet.
The B.sub.1 field is generated using an RF coil. Ideally, the B.sub.1 field is of uniform strength throughout the volume of matter to be imaged or analyzed. For in-vivo applications "whole body" coils are used for this purpose. While such coils are capable of generating a relatively uniform B.sub.1 field throughout a volume of body tissue, they are relatively complex in operation, expensive and undesirably large for many applications. Even these complex coils exhibit B.sub.1 inhomogeneities that lead to the appearance of B.sub.1 artifacts in MRI or MRS. Pick-up coils are employed to detect the RF band emissions stimulated by the transient B.sub.1 field. In applications where a high signal-to-noise ratio is desired, separate pick-up coils of smaller dimension that are placed directly over the region of interest are employed. Typically, relatively simple and inexpensive "surface coils" are used for this purpose. A surface coil is merely a wire loop typically having a diameter in the range of an inch to twelve inches. The pickup coils must be tuned to the same frequency as the whole body RF coil. However, if two coils tuned to the same frequency are within close proximity of each other, unless their B.sub.1 fields are completely orthogonal at each point in space, they couple to each other and alter their tuning frequencies. Therefore, when separate coils are used for RF transmission and signal detection, they must be decoupled from the RF coil in order to avoid confusing pulsed RF with the resulting return emissions from the body matter. Although this arrangement is technically feasible, high degrees of decoupling are difficult to attain and add an extra measure of complexity and artifact source to the system.
As compared to whole body coils such as birdcage coils and saddle coils, surface coils are relatively easy to work with and inexpensive. Moreover, because they do not require the relatively large geometries of whole body coils they can be configured to generate RF and detect signals only from the region of interest, eliminating the noise generated by all other regions in the sample. This can be very beneficial when attempting certain types of imaging or spectroscopic analysis. Furthermore, if the same coil can be used to both generate the RF pulses and sense the return emission, the need for decoupling and the corresponding artifact source is greatly reduced. Thus, it has been recognized for some time that it would be highly desirable if surface coils could be used to generate the required RF pulses as well as detect the returning emissions, particularly for localized imaging and spectroscopy. By their very nature, however, surface coils are unable to generate a uniform B.sub.1 field through any appreciable or useful volume of space; the magnitude of the B.sub.1 field generated by a surface coil is strongly dependent on spatial coordinates. As a consequence, surface coils have not been used for RF transmission in applications that require homogeneous B.sub.1 fields such as image construction or localized spectroscopy using tailored pulses in conjunction with pulsed-field gradients.
It is the inability to perform uniform 90 degree and 180 degree rotation of magnetic spins about a well defined axis which is the specific deficiency of surface coils when used for B.sub.1 field generation. Provided that such rotations could be performed despite B.sub.1 field inhomogeneities, RF coils of many different geometries could be used for both RF generation and signal detection. As previously mentioned, even coils with complex designs (such as the Helmholtz or the birdcage coil) have inhomogeneities in their B.sub.1 profiles. Pulses that induce uniform rotations despite such inhomogeneities would improve the performance of these coils and eliminate artifacts that arise because of them.
Others have suggested two frequency and amplitude modulated pulses which can be generated by a surface coil to rotate z-magnetization to the -z axis or the transverse plane of the rotating reference frame. See MS Silver, R. I., Joseph, and D. I. Holt, J. Magn. Reson 59, 347 (1984) and Mr. Bendall and D. J. Pegg, J. Magn, Reson. 67, 376 (1986). One of the unique features of these pulses is their relative insensitivity to large variations in the B1 field magnitude. However, these pulses cannot execute 90 degree or 180 degree plane rotations with a well defined phase. In other words, they cannot rotate by 90 or 180 degrees all magnetization vectors that are contained in a given plane. Therefore, while they can be employed for signal excitation or inversion, they cannot be used for refocusing (which requires a 180.degree. plane rotation pulse) or rotation of magnetization vectors from the transverse plane onto the z-axis (90.degree. plane rotations). Many imaging and spectroscopic pulse sequences require refocusing and 90.degree. plane rotations.
As set forth below, the present invention provides a method for generating RF pulses that perform the sought after 90.degree. and 180.degree. planar rotations even when B.sub.1 fields are highly non-uniform. Accordingly, the present invention provides a means for using surface coils or coils of any other geometry that possess variations in their B.sub.1 field profiles for successful imaging or spectroscopy applications. When used with more complex coils, these pulses will improve the coil performance by eliminating problems that arise due to the small but non-negligible inhomogeneities in the B.sub.1 profile of these coils. In addition, the present invention also provides a method for modulating the amplitude and frequency of adiabatic pulses to control the homogeneity of the B.sub.1 field.